Method for measuring flow noise of water over a hydrophone

ABSTRACT

A method for measuring water-flow noise over a hydrophone comprising: coupling the hydrophone to a distal end of a hydrodynamic-drag-reduced beam, which has a proximal end that is rotatably connected to a frame; pivoting the beam through water; and recording the water-flow noise generated by the water flowing around the hydrophone.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 11/512,538, filed 30 Aug. 2006 now U.S. Pat. No. 7,283,425titled “Apparatus For Measuring Flow Noise of Water Over a Hydrophone,”which is incorporated herein by this reference.

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention (Navy Case No. 98889) is assigned to the United StatesGovernment and is available for licensing for commercial purposes.Licensing and technical inquiries may be directed to the Office ofResearch and Technical Applications, Space and Naval Warfare SystemsCenter, San Diego, Code 2112, San Diego, Calif., 92152; voice (619)553-2778; email T2@spawar.navy.mil. Reference Navy Case Number 98889.

BACKGROUND OF THE INVENTION

Current practices of measuring the flow noise of water over a hydrophone(water-flow noise) involve allowing the hydrophone to free-fall throughwater or pulling the hydrophone through water with a boat. Allowing thehydrophone to free-fall through water limits the speed at which noisemeasurements may be taken to the terminal velocity of the hydrophone.Also, when towing the hydrophone through water with a boat, noise fromthe boat and/or the towlines adversely affects the water-flow noisemeasurement.

A need exists for a method of measuring water-flow noise at a variety offlow speeds with a reduced amount of unwanted noise as compared tocurrent practices.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the several views, like elements are referenced using likereferences.

FIG. 1 a is a front view of an embodiment of an apparatus for measuringwater-flow noise.

FIG. 1 b is a front view of an embodiment of an apparatus for measuringwater-flow noise in an equilibrium position.

FIG. 2 is a sectional, perspective view of an embodiment of a section ofan apparatus for measuring water-flow noise.

FIG. 3 is a cross-sectional view of an embodiment of a pivotal beam ofan apparatus for measuring water-flow noise.

FIG. 4 is a front view of an embodiment showing an attached weight.

FIG. 5 a is a front view of an embodiment of a beam retriever.

FIG. 5 b is a front view of another embodiment of a beam retriever.

FIG. 6 a is a front view of an embodiment of an angular displacementsensor.

FIG. 6 b is a side view of another embodiment of an angular displacementsensor.

FIG. 7 a is a front view of another embodiment of an angulardisplacement sensor.

FIG. 7 b is a side view of another embodiment of an angular displacementsensor.

FIG. 7 c is a rear view of an embodiment of a counter assembly.

FIG. 8 is a front view of another embodiment of an angular displacementsensor.

FIG. 9 is a rear view of another embodiment of a beam retriever.

FIG. 10 is a front view of another embodiment of an apparatus formeasuring water-flow noise.

FIG. 11 is a block diagram of an apparatus for measuring water-flownoise.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 a illustrates a water-flow noise measuring apparatus 8 comprisinga frame 10, a beam 20 with a distal end 29 and a proximal end 21, ahydrophone 30, an angular displacement sensor 40, and a computer 50.Pivotably coupling the proximal end 21 to the frame 10 at a pivotal axis25, and then allowing the distal end 29, to which the hydrophone 30 iscoupled, to travel in an arc through water allows the water-flow noiseto be measured as a function of the flow rate of the water over thehydrophone 30. The flow rate of water over the hydrophone 30 may beequated with the tangential velocity ν of the hydrophone 30 as beam 20pivots through the water. The tangential velocity ν of the hydrophone 30may be calculated according to the equation:ν=ω*R ₁Where ω is the angular speed (rad/s) of beam 20, and R₁ is the distancebetween the hydrophone 30 and the pivotal axis 25.

The frame 10 may be a structure constructed of any material capable ofsupporting the weight of beam 20 as transferred to frame 10 via pivotalaxis 25. Frame 10 may also support the angular displacement sensor 40.The frame 10 may be capable of floating on an air-water interface, ormay be secured to land in such a way as to allow beam 20 to pivotthrough water. A few example embodiments of frame 10 include a pier, aboat, a buoy, or a structure that may be mounted on a pier or boat suchthat beam 20 may pivot through water.

The hydrophone 30 may be mounted to beam 20 at a predetermined distanceR₁ from proximal end 21 of beam 20. The hydrophone 30 is capable ofdetecting the noise generated by water flowing over the hydrophone 30 asbeam 20 pivots through the water and then generating water-flow noisedata 35, as shown in FIG. 11, which is then received by a computer 50.In one embodiment, the distal end 29 of beam 20 may be allowed to dropfrom a position in which beam 20 is parallel to the water line untilbeam 20 comes to rest at an equilibrium position, as shown in FIG. 1 b.During such a drop, the distal end 29 of beam 20 will start from zerotangential velocity, reach a maximum free-fall tangential velocity, andslow to zero tangential velocity as it comes to rest at an equilibriumposition.

FIG. 2 illustrates an embodiment of the beam 20, wherein beam 20 has ahydrodynamic drag reducing shape and a hollow core 22. The hydrodynamicdrag reducing shape may be any shape with a relatively smooth,streamlined surface such as a hydrofoil. A suitable hydrodynamic dragreducing shape, by way of example, for beam 20 may be an aircraft strutwith an elliptical cross section, as shown in FIG. 3, with a majordiameter D₁ of about 3 inches and a minor diameter D₂ of about 1 inch.FIG. 2 also shows signal wires 33 routed through the hollow core 22 ofbeam 20. The signal wires 33 are disposed to carry water-flow noise data35, generated by the hydrophone 30, as shown in FIG. 11, to the computer50 for storage and/or processing. Routing the signal wires 33 throughthe hollow core 22 of beam 20 allows beam 20 to pivot through waterwithout creating any additional noise-producing turbulence due toexposed wires. The beam 20 may be made of any material that is capableof resisting anything more than insignificant deformation in thepresence of hydrodynamic flow-induced stress. For example, beam 20 maybe made of aluminum, wood, steel, plastic, or a fiber/resin composite,to name a few.

FIG. 2 also illustrates how beam 20 may be comprised of a plurality ofsegments S₁-S_(n) coupled to each other, where n may be any positiveinteger. The length L₁ of the beam 20 as well as the distance R₁ betweenthe hydrophone 30 and the pivotal axis 25 may therefore be altered bycontrolling the number of segments S₁-S_(n) comprising beam 20. Forexample, a segment may be added or removed to increase or decreaserespectively the length L₁ of the beam 20; and the hydrophone 30 may bemounted between any two of segments S₁-S_(n), thus altering the distanceR₁ between the hydrophone 30 and the pivotal axis 25. Adjustments to thedistance R₁ between the hydrophone 30 and the pivotal axis 25 may beused to adjust the speed of the hydrophone through the water. Thegreater the distance R₁ the faster the effective flow rate of water overthe hydrophone 30. The lengths l₁-l_(n) of each of the segments S₁-S_(n)may be any desired length. By way of example, five feet may be asuitable dimension for each of lengths l₁-l_(n).

FIG. 4 shows a weight 60 operably coupled to the distal end 29 of beam20. Unlike free-falling objects of differing masses in a vacuumenvironment that fall at the same rate, objects of similar volume and ofdiffering masses will sink in water at differing rates due to the effectof buoyant and drag forces. Adding weight 60 to the distal end 29 servesto increase the free-fall speed of descent of beam 20 as it pivotsthrough the water about pivotal axis 25. The weight 60 may be made ofany material with a density greater than that of the beam 20. The massof the weight 60 may be altered depending on the desired tangentialvelocity ν of hydrophone 30. As a general, free-fall rule, the greaterthe mass of weight 60, the greater the tangential velocity ν ofhydrophone 30 up to a terminal velocity of beam 20. Also, the weight 60may have a hydrodynamic-drag-reducing shape such that weight 60generates a minimal amount of noise-causing turbulence. By way ofexample, one embodiment of weight 60 involves filling of beam segmentS_(n) at the distal end 29 with a desired amount of lead.

FIG. 5 a illustrates another embodiment of a water-flow noise measuringapparatus 8 further comprising a beam retriever 70, which is operablycoupled to the distal end 29. The beam retriever 70 may be a line ofsufficient strength to allow a user to pull beam 20 up towards thewaterline such as a nylon rope. In another embodiment, as shown in FIG.5 b, the beam retriever 70 may be a buoy operably coupled to the distalend 29. As a buoy, the beam retriever 70 may be inflatable and/or have ashape that minimizes the amount of hydrodynamic drag created as the beamretriever 70 moves through the water such as a hydrofoil. If the beamretriever 70 is inflatable, an air inflation line 77 may be routedthrough hollow core 22 to a compressed air supply 78, as shown in FIG.3.

The angular displacement sensor 40, operably coupled to the proximal end21 of the beam 20 and the frame 10, is capable of providing displacementdetection signals 45 to computer 50, as shown in FIG. 11, from which thecomputer 50 may determine the angular speed ω of the beam 20 withrespect to the frame 10. In one embodiment, the angular displacementsensor 40 may be a non-contact optical sensor capable of opticallydetecting the angular displacement of the beam 20 as it pivots aboutpivotal axis 25 without physically contacting beam 20. By way ofexample, FIGS. 6 a-6 b illustrate an embodiment of the angulardisplacement sensor 40, comprising a first sheave 41, which may have yoptical pick-up points 49 circumferentially positioned on an outer edgeof first sheave 41, as shown in FIG. 6 b, where y may be any positiveinteger. The first sheave 41 may be mechanically coupled to beam 20 viapivotal axis 25 such that the angular displacement of beam 20 and firstsheave 41 may be about equal. The embodiment shown in FIG. 6 a alsoshows an optical sensor 47 disposed to detect an optical pick-up point49 and generate a displacement detection signal 45 for each opticalpick-up point detected. Displacement detection signals 45 may then bereceived by the computer 50 via transmission line 92. The transmissionline 92 need not be a physical line but may be a wireless connection.Every displacement detection signal 45 received by the computer 50 in agiven time interval Δt may be counted by the computer 50. For theembodiment shown in FIGS. 6 a-6 b, the angular speed ω of beam 20 may becalculated according to the equation:ω=(2*π*x)/(y*Δt)Where π is the well-known constant representing the ratio of a circle'scircumference to its diameter, or 3.14159265 . . . ; Δt equals a giventime interval in seconds; x equals the number of displacement detectionsignals 45 received by the computer 50 in the time interval Δt; and yequals the total number of optical pick-up points 49 on the first sheave41. Although the embodiment of the angular displacement sensor 40 shownin FIGS. 6 a and 6 b includes optical pick-up points 49 on a firstsheave 41, it is to be understood that the optical pick-up points 49 maybe on any structure, made of any material, that is capable of presentingthe optical pick-up points 49 to the optical sensor 47 such that opticalsensor 47 may detect the optical pick-up points 49 as the structurerotates about the pivotal axis 25.

FIGS. 7 a-7 c are depictions of another embodiment of the angulardisplacement sensor 40 comprising a first sheave 41 having a diameterD₃, a second sheave 42 having a diameter D₄, a belt 43, and a counterassembly 44. The first sheave 41 may be mechanically connected to theproximal end 21 of beam 20 such that the first sheave 41 may have thesame angular speed ω as beam 20. As shown in FIG. 7 a, the second sheave42 may be rotatably connected to frame 10, and mechanically coupled tothe first sheave 41 by the belt 43. As beam 20 and the first sheave 41rotate through the water, the rotational motion is transferred to thesecond sheave 42 via belt 43. The counter assembly 44 may comprise atoothed gear 46 and a gear-tooth counter 48, as shown in FIGS. 7 b and 7c. In one embodiment, the toothed gear 46, having m teeth 98, may beoperably coupled to the second sheave 42 as shown in FIGS. 7 b and 7 csuch that rotational motion of the second sheave 42 may be translated totoothed gear 46 via a connecting shaft 95. The gear-tooth counter 48 maybe connected to frame 10 by adjustable fixture 10 a. The gear-toothcounter 48 may be positioned such that the gear-tooth counter 48 maycount the teeth 98 of gear 46 as gear 46 rotates. By way of example, thefirst sheave 41 may have a diameter D₃ of about 15 inches, the secondsheave 42 may have a diameter D₄ of about 3 inches, the belt 43 may be around rubber belt, the toothed gear 46 may have 60 teeth 98, and thegear-tooth counter 48 may be a magnetic proximity sensor capable ofdetecting each gear tooth 98 as it passes the gear-tooth counter 48, asshown in FIG. 7 c. The gear tooth counter 48 may be capable ofgenerating a gear tooth detection signal for every gear tooth 98detected. Each gear tooth detection signal, transmitted to the computer50 via transmission line 92, may be counted by the computer 50. For theembodiment shown in FIGS. 7 a-7 c, the angular speed ω of beam 20 may becalculated according to the equation:ω=(π*x*D ₄)/(2*m*Δt*D ₃)Where π is the well-known constant representing the ratio of a circle'scircumference to its diameter, or 3.14159265 . . . ; Δt equals a giventime interval in seconds; x equals the number of gear tooth detectionsignals received by computer 50 the time interval Δt; m equals the totalnumber of teeth 98 in gear 46; D₃ is the diameter of the first sheave41; and D₄ is the diameter of the second sheave 42.

FIG. 8 shows another embodiment of the angular displacement sensor 40,wherein the angular displacement sensor 40 may be a Doppler currentmeasuring device located near the hydrophone 30, capable of directlymeasuring the flow rate of the water flowing past the sensor. TheDoppler current measuring device may also have a shape that reduceshydrodynamic drag. The angular displacement sensor 40 is not limited tothe embodiments described above but may be any comparable device thatmay be used to determine the angular speed ω of beam 20 and/or theeffective flow rate of water over hydrophone 30.

FIG. 9 depicts another embodiment of beam retriever 70, wherein thetoothed gear 46 may be turned manually or by a motor 77 to aid in theretrieval of beam 20 from the equilibrium position. In the embodimentdepicted in FIG. 9, the rotation of the toothed gear 46 counterclockwisemay serve to move beam 20 back towards the water line. Rotating toothedgear 46 counterclockwise may serve to increase the angular speed ω ofbeam 20 thus achieving tangential velocities greater than could beachieved by permitting beam 20 to pivot freely through the water aboutpivotal axis 25.

While the hydrophone 30, coupled to beam 20, pivots through the water atvarious speeds, water-flow noise data 35 generated by the hydrophone 30and displacement detection signals 45 generated by the angulardisplacement sensor 40 may be collected and stored by the computer 50.The computer 50 may then determine the tangential velocity ν of thehydrophone 30, which is about equal to the flow rate of water over thehydrophone 30. The water-flow noise data 35 and the tangential velocityν may be used by the computer 50 to generate an output signal 52representing the water flow noise data 35 as a function of thetangential velocity ν, as shown in FIG. 11. The output signal 52 maythen be plotted on display 55. Although the output signal 52 is shown inFIG. 11 as being transmitted from the computer 50, it is to beunderstood that the output signal 52 may be transmitted to a display,and/or used internally as an input to another function performed by thecomputer 50.

FIG. 10 illustrates an alternative embodiment of the water-flow noisemeasuring apparatus, wherein the orientation of the hydrophone 30 withrespect to the water flow is variable. For example, FIG. 10 shows thehydrophone 30 rotated 180° about a long axis 27 from the orientation ofthe hydrophone 30 shown in FIG. 1 a. Different orientations of thehydrophone 30 with respect to the water flow may be accomplished bypositioning the hydrophone 30 to any desired orientation with respect tobeam 20.

From the above description of the method for measuring water-flow noise,it is manifest that various techniques can be used for implementing theconcepts of the method without departing from its scope. Moreover, whilethis method for measuring water-flow noise has been described withspecific reference to certain embodiments, a person of ordinary skill inthe art would recognize that changes may be made in form and detailwithout departing from the spirit and the scope of the claims. Thedescribed embodiments are to be considered in all respects asillustrative and not restrictive. It should also be understood that themethod for measuring water-flow noise is not limited to the particularembodiments described herein, but is capable of many rearrangements,modifications, and substitutions without departing from the scope of theclaims.

1. A method for measuring water-flow noise over a hydrophone comprising:coupling the hydrophone to a distal end of a hydrodynamic-drag-reducedbeam, a proximal end of the beam being rotatably connected to a frame,pivoting the beam through water, and recording the water-flow noisegenerated by the water flowing around the hydrophone.
 2. The method ofclaim 1, further comprising: recording tangential velocity of thehydrophone while the beam rotates through the water, and correlating thevelocity and the water-flow noise.
 3. The method of claim 1, furthercomprising altering the orientation of the hydrophone with respect tothe water flow.
 4. A method for measuring water-flow noise over ahydrophone comprising: a. pivoting a hydrodynamic-drag-reduced beamthrough water around a submerged pivotal axis, wherein the beam'sproximal end is pivotably coupled to the pivotal axis and the hydrophoneis coupled to the beam's distal end, and b. measuring the water-flownoise from the hydrophone as a function of the flow rate of the waterover the hydrophone.
 5. The method of claim 4, wherein step b. furthercomprises: generating water-flow noise data with the hydrophone, andtransmitting the water-flow noise data to a computer.
 6. The method ofclaim 5, wherein the pivoting step further comprises allowing the beamto free-fall from a water-line-parallel position until the beam comes torest at an equilibrium position.
 7. The method of claim 6, furthercomprising: c. retrieving the beam after the beam has reached theequilibrium position.
 8. The method of claim 7, further comprising d.varying a parameter and repeating steps a-c.
 9. The method of claim 8,wherein the step of varying a parameter comprises increasing the lengthL₁ of the beam such that the distance R₁ between the hydrophone and thepivotal axis is increased.
 10. The method of claim 8, wherein the stepof varying a parameter comprises decreasing the length L₁ of the beamsuch that the distance R₁ between the hydrophone and the pivotal axis isdecreased.
 11. The method of claim 8, wherein the step of varying aparameter comprises altering the tangential velocity ν of the hydrophoneby adding a weight to the distal end of the beam.
 12. The method ofclaim 8, wherein the step of varying a parameter comprises altering theorientation of the hydrophone with respect to the water flow.
 13. Themethod of claim 8 wherein the flow rate of water over the hydrophone isdetermined by the following steps: transmitting angular displacementdetection signals representative of the angular displacement of the beamfrom an angular displacement sensor to the computer; determining anangular speed ω of the beam with respect to a frame with the computer;determining the tangential velocity ν of the distal end of the beam withthe computer; and equating generally the flow rate of water over thehydrophone to the tangential velocity ν.
 14. The method of claim 13,wherein the angular displacement sensor is a non-contact optical sensorcapable of optically detecting the angular displacement of the beam asit pivots about the pivotal axis.
 15. The method of claim 8 wherein theflow rate of water over the hydrophone is measured with a Dopplercurrent measuring device located near the hydrophone.
 16. The method ofclaim 13, further comprising generating an output signal with thecomputer, wherein the output signal represents the water flow noise dataas a function of the tangential velocity ν.
 17. The method of claim 16,further comprising plotting the output signal on a display.
 18. Themethod of claim 5, wherein the pivoting step comprises propelling thebeam through the water with a motor.